Dendrites constitute as much as 90% of the receptor surface offered by a neuron for contact with other neurons, and are, therefore, important determiners of the functional capabilities of single neurons and sets of neurons. A wide variety of studies have established that dendritic change throughout the entire lifespan of the organism by growing and regressing. The early stages of development have been a particular focus of previous studies and have suggested that dentditic change may be related to afferent activity and that synaptic contacts are made on recently formed dendritic surface. In the correlative Golgi-electronmicroscopic studies proposed here, the major focus will be on later stages of the life span. We hypothesize that the same principles relating afferent axons to dendritic change during early development are applicable to the similar types of dendritic change seen during aging. We plan to test this hypothesis by first determining the probable state of change of dendrites by quantitative measures of Golgi stained neurons at the light microscopic level. We will select from these cells those whose dendrites are most likely to be either growing or regressing. These dendrites will be resectioned for electron microscopy. It will be possible to identify the selected Golgi stained dendrites in the electron micrographs by virtue of their having been selectively stained, and quantitatively assess the axons, synapses and other structures surrounding the tips (the most plastic portion) of the selected dendrites. The probability of finding growing or regressing dendrite tips will be optimized by examining animals at different stages in the life span (which may also allow us to see a temporal relation between changes in surrounding structures and dendritic changes) and by experimental manipulations: lesions of nucleus basalis (the major source of cholinergicaffernets to cortex), and environmental enrichment. These studies will allow us to test the hypothesis that afferent axonal input is an important determiner of the state of dendrites in aging, as it is in early development.